Everything is explained on chart .. the start of the possible harmonic happens at the end of the C leg to find that you need to identify and keep an eye on the B leg
Train your eyes to identify the Structure. Latter comes the logic and understanding.
DJIA is at the PRZ of a Bearish Harmonic Butterfly which suggest it could retrace till 27000. Pattern would be negated above 28600
Harmonic Trading Patterns work as reversal signals based on Fibonacci ratios, Butterfly is one such pattern.